The recent discovery of new classes of small regulatory RNA species and the factors that control their[unreadable] biogenesis and function has revealed unexpected new mechanisms of gene regulation that affect normal[unreadable] development and, likely, cancer development. These discoveries have also led to the generation of powerful[unreadable] new tools for probing gene function in normal cells and in disease. This Project is focused on the investigation[unreadable] of the role of miRNAs in tumorigenesis and cell transformation as well as the use of RNAi to examine the[unreadable] function of genes in these processes. This will be investigated through a series of gene targeting[unreadable] experiments in the mouse directed at the mir-17approximately 92 cluster and its paralogs, which have been implicated in[unreadable] human tumorigenesis. These studies will also facilitate the discovery of the targets of these miRNAs. The let-[unreadable] 7 family of miRNAs, which have been reported to regulate Ras function, will be investigated using cell-based[unreadable] and in vivo methods. The role of miRNAs in tumorigenesis will be further tested by miRNA profiling[unreadable] experiments in a well-defined mouse model of lung cancer progression and through mutation of components[unreadable] of the miRNA processing machinery. Finally, this Project will seek to develop improved vectors and expression[unreadable] systems for constitutive and conditional shRNA expression in vivo. A specific application of this technology[unreadable] will be to test the function of a set of candidate tumor suppressor genes in lung cancer.[unreadable] Relevance statement: The elucidation of the molecular genetic events that underlie tumor development is[unreadable] critical for the effective treatment and prevention of cancer. This research is directed at understanding the[unreadable] function of a new class of regulatory molecules, termed miRNAs, in the development of cancer. The[unreadable] research will also lead to the development of powerful new tools to study cancer-relevant genes and[unreadable] pathways.